Engagement of the T cell receptor by ligand results in a cascade of biochemical events that result in T cell proliferation and T cell effector responses such as cytolysis and cytokine secretion. The broad goals of this proposal are to define some of these early T cell signaling events and to determine how these events are initiated. As these signaling events play critical roles in almost all immune responses, a greater understanding of these events have clear practical relevance. Early signaling molecules would be excellent targets for therapies that aim to either enhance or suppress a specific immune response. Activation of a tyrosine kinase is thought to be the primary signaling event transduced by the T cell receptor. Although the identity of the tyrosine kinase activated is unknown, one candidate is the tyrosine kinase, p59fyn. It is associated with the T cell receptor, and can enhance signaling of T cells when it is overexpressed in transgenic mice. The specific goals of this proposal are to perform a structural characterization of the p59fyn/T cell receptor complex and to investigate the mechanism(s) used by p59fyn to signal in T cells. We propose to continue our previous work defining important structural features of the p59fYn/T cell receptor interaction (Aim #1). We will then determine the functional significance of this interaction by determining whether p59fyn is required for T cell receptor signaling or signaling by chimeric proteins that contain only single components of the T cell receptor (Aim #2). To define the specific roles of p56lck and p59fyn in T cell activation, we will define the domains of both proteins that give specificity to their actions (Aim #3). The function of these domains will then be examined by trying to identify proteins that interact with these domains (Aim #4).